1. Field of the Invention
The present invention relates to a microwave applicator device for the treatment of sheet or lap products, of the type comprising a housing defining a parallelepipedal waveguide cavity with dimensions a x b.times.L in an orthogonal coordinate system Ox, Oy, Oz, the the housing being aligned along Oz and provided with slots for passing the product to be treated into the cavity along a plane parallel to the Ox, Oz plane, and means for exciting the cavity in transverse electric mode (TE), in order to create an electric field internal to the cavity along a direction substantially parallel to Ox.
By microwaves must be understood waves with frequencies lying between 0.3 GHz and 300 GHz, and more particularly those situated in the S-band [1.55 GHz to 5.2 GHz].
The invention finds a particularly important, though not exclusive, application in the field of the drying of sheet or thin lap products, that is to say of thickness less than of the order of 20 mm, especially in the fields of papermaking, printing (drying of inks), for the preparation of hides in the leather industry, or for the drying of damp powders laid out in laps. Microwaves of standard frequency equal to 2.45 GHz will be especially advantageously used.
However the invention can quite obviously be applied to other treatments and especially to heat treatments with differing microwave frequencies and on products in sheets of greater thickness.
2. State of the Art
Devices for microwave treatment of sheet products are already known. They most often call upon, either waveguide housings with so-called "winding", bent structure, or parallelepipedal waveguide housings, of the type defined above, slotted on the large sides for the passing of the product to be treated, this avoiding disturbance to the current lines of the fundamental mode of the electric field.
These known solutions allow a fairly homogeneous treatment, but, being able to employ only a low-intensity electric field, are either bulky and complicated (in the case of "winding" structures), or limited in their usage, since not allowing a working period sufficient for the desired treatment of the product (in the case of slotted guides). In the latter case, in fact, the known parallelepipedal "waveguide" housings possess a transverse cross-section of low width a; for example, the standard dimensions a.times.b of the transverse cross-sections of housings are 4.3 cm.times.8.6 cm in Europe, and 3.4 cm.times.7.2 cm in the United States. The sheet product which advances in the transverse sense through the slots of the housing, can therefore remain for only a limited period in the cavity excited in TE mode.
There could, quite obviously, be a temptation to raise the residence time by slowing the speed of advance, or even by stopping the product in the housing, for a specified period. However such a solution would be to the detriment of the homogeneity of treatment also desired. In fact, in the case of an applicator device with advance, it is possible to be content with an approximately uniform electric field over the whole width of the carrier band, without worrying about the direction of the advance, since there will be a statistical homogenising of the energy absorbed during traversal of the housing. This is no longer the case for a static applicator device.
To remedy the disadvantage of the low-intensity electric field and reduce the bulkiness of the applicator device, it has been possible to call upon a resonant applicator whose electric field is more intense for a given microwave power.
In fact, in the case of a resonant wave, the electric field is, as is known, multiplied by the square root of the overvoltage, the overvoltage being defined as the ratio between the total energy stored in the resonator and the energy dissipated per period (modulo 2 .pi.).
However, use of a resonant applicator has the disadvantage of no longer allowing a homogeneous treatment over the whole width of the sheet product to be treated since the electric field possesses intensity nodes and antinodes.
To remedy this disadvantage, a system has been proposed consisting of at least two identical resonant waveguide cavities through which the sheet to be treated advances, and which are mutually offset by (1/N).times..lambda.g/2, in order to distribute the effect of the field maxima over the whole width of the product [FR No. 2,523,797].
If the latter solution is satisfactory, it can in particular be further improved. In fact, on the one hand it requires the presence of several guide cavities, on the other hand, it is known that resonant cavities often pose particular matching problems.
In fact, their functioning is closely dependent upon load variations, and a regulating of the frequency to the variations in intensity of the field is often necessary for a precise tuning to the resonance.